(a) Field of the Invention
This invention relates to the production of titanium boride. More specifically, the present invention is directed to the production of TiB.sub.2 composite material for wear-resistant coatings, and parts. The invention also relates to the production of titanium boride powder.
(b) Description of Prior Art
Among the different ceramic compounds, TiB.sub.2 is one of the most interesting because of its exceptional characteristics. The transition metal diboride TiB.sub.2 combines such important properties as high hardness, high melting point, good electrical or thermal conductivity and good corrosion resistance. These properties are responsible for the fact that TiB.sub.2 is attractive in various fields of engineering where parts must have wear-resistance as well as good oxidation resistance or high thermal resistance in different media.
Up to now, TiB.sub.2 has been produced by directly reacting titanium and boron or by the so-called borocarbide method, i.e. by heating mixtures of Ti, B.sub.4 C and B.sub.2 O.sub.3 or TiO.sub.2 at temperatures of 1800.degree. C. to 2000.degree. C. These methods have important drawbacks since expensive starting elements such Ti, B or B.sub.4 C are used. Furthermore, these methods cannot produce TiB.sub.2 composite materials containing a fine dispersion of interesting elements such as Fe, Ni or Co, Al, Mo, Cr and Cu.
It is also known that the applications of TiB.sub.2 are limited by the brittleness of pure TiB.sub.2 or because difficulties of fabrication are involved in order to obtain dense coatings from pure TiB.sub.2 powders. It is well recognized that the mechanical properties of TiB.sub.2 need to be improved in order to enable this material to be used under industrial conditions. The best way to enhance the mechanical properties of TiB.sub.2 is to associate this material with metallic binders. This was generally performed by using mixtures of TiB.sub.2 and metallic powders, mainly iron and nickel powders.
These mixtures can then be used to produce parts based on TiB.sub.2 by sintering or hot pressing. They can also be used to produce thermal sprayed coatings but this requires a careful control of mixing and the use of fine powders in order to achieve a good distribution of each constituent. Furthermore, a limiting feature of the thermal sprayed coating is the need of melting powder during its travel through the flame. The high melting point of borides, particularly titanium boride, limits the use of thermal sprayed coatings of these compounds. Indeed, the temperature required to melt TiB.sub.2 is so high and the times of residence of particles within the flame are not long enough to produce an adherent layer. Because of their high melting points, coatings based on TiB.sub.2 have not been satisfactory achieved.
U.S. Pat. No. 4,014,688 issued to Horst Schreiner et al on Mar. 29, 1977 discloses the fabrication of contact material for high-power vacuum circuit breaker. This material consists of an alloy having a base metal and alloying metals which form a eutectic with the base metal used. Iron and titanium are mentioned as possible base metal whereas boron is mentioned as alloying element. The contact materials described by Schreiner et al consist of a base metal with dispersed second phases obtained by formation of a eutectic. These contact materials are hypo- or hyper-eutectic alloys and are not ceramic materials. The proportion of iron, titanium and boron used by Schreiner et al is up to about 90% iron, up to about 90% titanium and about 1% boron.
Canadian Pat. No. 686,187 which issued on May 12, 1964 is directed to a method of preparing a titanium powder containing titanium monoboride, which is different from titanium diboride. This powder can be consolidated to produce parts having a titanium matrix having titanium monoboride dispersed in it. However, the maximum content in titanium monoboride particles is limited to about 30 vol. %. Furthermore, there is no mention of other metallic matrix like Fe, Ni, etc.
Canadian Pat. No. 1,003,246 which issued on Jan. 11, 1977 relates to a wear-resistant composite materials. They consist of a dispersion of coarse (0.3 to 1 mm) particles within a matrix of copper or nickel. These composites are heterogenous materials containing about 50 vol. % of rich titanium and boron bearing particles. The hardfacing is the only method suitable to produce overlays based on those materials. They are not constituted of fine dispersion of TiB.sub.2 in a metallic matrix.
Canadian Pat. No. 1,110,881 relates to another wear-resistant product, which is made of ironmolybdenum boride. It is not based on TiB.sub.2.